PSI - Issue 12

Gabriel Testa et al. / Procedia Structural Integrity 12 (2018) 589–593 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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with the reduction of “buy -to- fly” ratio. Innovation is also expected for unmanned aerial vehicles , Herderick (2011). The increasing expectations on such technologies brought out the importance of the “quality” , not only intended as dimensional accuracy and lack of defects, but also in terms of effective mechanical properties, as a critical factor. At present, published studies on the correlation between the production process, part shape and effective properties are limited and not systematic. Lewandowski and Seifi (2016), in a detailed review of mechanical properties of additively manufactured materials, concluded that published data on standard samples are limited while papers addressing fundamental issues such as fracture resistance, response to impact, creep and multiaxial fatigue are missing. In the case of AM, the printing process in association with the shape of the component, leads to peculiar microstructure and residual stress/strain fields that determine the performance in service. The role of the microstructure not necessary becomes particularly evident in traditional quasi-static characterization testing, where the uniform stressed material volume is significantly larger than the microstructural length scale. Alternatively, high strain rate testing provides a unique mean to probe the role of microstructure on the material strength. In this work, additively manufactured Ti 6Al-4V, printed using direct metal laser sintering process was tested under high velocity impact conditions. The scope of the work was to investigate at which impact velocity ductile damage initiates, in comparison to that of wrought material, by means of Taylor impact tests. Preliminary results showed that the onset damage velocity is significantly lower for the AM material. This behavior is probably due to the presence of microvoids resulting from the printing process that seems to trigger anticipated formation of shear bands. The material under investigation is commercial Ti-6Al-4V. This is an alpha-beta titanium alloy with a high strength-to-weight ratio and excellent corrosion resistance. It is one of the most commonly used titanium alloys, widely used in aerospace and biomechanical applications. Additively manufactured (AM) Ti-6Al-4V was printed using direct metal laser sintering (DMLS). This process is a rapid prototyping, AM technique designed to use a high power-density laser to melt and fuse metallic powders together. DMLS has many benefits over traditional manufacturing techniques. Since components are built layer by layer, it is possible to design internal features and passages that could not be cast or machined otherwise, Fig. 1. Cylinders 10.8 mm in diameter and 54 mm long were 3D printed. Each batch included samples oriented along x, y, z and xy 45° and xz 45° directions to investigate possible anisotropy effect due to the printing direction. After printing, each batch was heat treated at 800°C (1470°F) for 4 hours in argon inert atmosphere. At the end of this process, the microstructure has a lamellar structure of primary  and  phase and  grain boundary. Microscopy investigation revealed that the microstructure is also characterized by the presence of distributed microvoids of variable shape and size, ranging from few up to hundreds microns. The presence of such voids was also confirmed by X-ray CT scan, Fig. 2. 2. Material and methods 2.1. Material

Fig. 1 Sketch of direct metal laser sintering process.